This invention relates generally to the field of ultrasonic welding of synthetic resinous planar materials using a rotating pattern roll and a relatively fixed horn, and more particularly to a means and method for maintaining uniformity of welds during substantially continuous operation.
In ultrasonic quilting, the heat generated to melt the fabric together is caused by the interference of sound waves. The heat is generated at the locus of points where a sound wave leaving the horn interferes with a wave that has been reflected by the pattern roll. The distance between the horn and the pattern roll thereby becomes fixed as a function of the wave length of the sonic energy. As a result, a variation of this gap distance of as little as 0.002 inch can be detected as a lighter or heavier weld.
In conventional running of ultrasonic sealing machines, the horn is forced down on a plurality of layers of material under varying degrees of air pressure, using different pressures to compensate for different thicknesses of fabric. In order to make this pressure gap condition constant for all portions of the pattern, the prior art practice has been to use patterns in which the sealing areas remain constant on the fabric passing beneath the anvil, so that there is a substantially constant power dissipation.
One manner in which pressure sensitivity may be avoided, so that a broader range of "unbalanced" patterns may be used, is to bring the horn down into pisition with much more air pressure than is normally required to make a proper weld, but to provide an adjustable mechanical stop which provides accurate maintenance of the proper gap.
When such a structure was first employed, it was discovered that the solution brought with it unforeseen complications. Conventional pattern rolls are not manufactured with sufficiently close tolerance to maintain the gap against a relatively fixed horn within plus or minus 0.001 inch as it rotates. When such a roll was manufactured, another, more serious, problem occurred. As the machine operates, a low grade heat builds up in the pattern roll which is of hollow configuration. Since the heat builds up in the welding areas, and is not dissipated at a sufficiently rapid rate, distortion occurs with thermal expansion, and results in bowing of the roll, so that it rotates eccentrically with high and low areas to vary the degree of welding accomplished from no weld at all to a burn through. Using a roller having an outer diameter of 10-1/4inches and a wall thickness of approximately one inch, and 132 inches long, maximum deflections from true have been as much as 0.012 inch. It has been determined that the distortion is caused not by a simple rise in temperature, but by the fact that the roller warms during continued use in an uneven manner as a result of practical impossibility to manufacture the pattern roll with a uniform wall thickness. The situation is further complicated by the fact that the points at which heat enters the mass of the roller is determined by the pattern itself, and the problem manifests itself when running "unbalanced" pattern, i.e. those patterns which do not distribute sealing energy in a relative even manner.